Algae hot spots raise concern over water

High levels of blue-green algae have been found in several areas of Sydney's drinking water supply, according to an audit of the catchment to be released today.

Some of the blooms, which are the result of high levels of nutrients in the water and can cause skin irritation and gastrointestinal problems, lasted for more than 100 days, leading to calls for an overhaul of Sydney's water storage system.

High levels of cryptosporidium and giardia, found in faecal material, were also detected in two areas, in the assessment of the 16,000 square kilometre catchment carried out by the Department of Environment and Conservation.

The results were taken from water that had not yet been filtered or treated and water quality generally met the requirements of Sydney Water and NSW Health over the two-year period of the audit, from 2001 to 2003.

The department's acting deputy director general, Simon Smith, said the health of the catchment had remained relatively stable since 2001.

"The less encouraging news is that some parts of the catchment remain degraded, with the vegetation on the banks of some waterways in poor health," Mr Smith said.

Vegetation acts as a natural filter for much of Sydney's drinking water supply in addition to the treatment it receives before being piped into people's homes.

The general manager of bulk water for the Sydney Catchment Authority, Ian Tanner, said the results were not alarming but would take some time to address.

Algal blooms are the result of high levels of nutrients, such as phosphate and nitrate, in the water. They can come from manure or soil sediments loaded with fertilisers, and thrive particularly in still, shallow bodies of water, particularly in warm weather.

The two areas where the blooms were of most concern were in the Shoalhaven and Wingecarribee catchments, which are regularly used to top up Sydney's water supply. There were six sites at which levels of algae were found to be four times higher than naturally occurring levels.

"If the toxicity of the bloom is significant, the water becomes unusable for either drinking or other direct contact," the audit warned. "Even at low concentrations, some blue-green algae can impart strong tastes and odours to the treated water."

Mr Tanner said the presence of the algal blooms was a result of agriculture in the area: "It's more of a nuisance value. Farmers need to look at the fertilisers they are using and also their methods of farming to prevent run-off from ending up in streams and rivers."

But Mr Smith said addressing the source of the nutrients, such as agricultural run-off, was not enough to clean up water storage systems.

Because the blooms thrived in warm, still water, more needed to be done to address the condition of water after it had been captured for storage, he said.

Work also needed to be carried out to make sure the vegetation around water bodies, which acts as a natural filter, was maintained.

Dams in protected areas, including Warragamba, were in good condition because of the vegetation that remained, Mr Smith said. "But further away the vegetation has gone, which removes one stage of the filtering process."